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Now, a new study in our closest ancestors suggests that we may also not be alone in our awareness that others may have different thoughts, experiences and views of the world than we do. The study, published this week in the journal PLOS ONE, aimed to prove this question of consciousness by looking at whether great apes recognize “theory of mind”—that is, the understanding that others have their own (presumably different) minds.

Psychologists are hampered in these kinds of studies by the frustrating fact that it's not possible to step into the mind of another person—or creature—to study how it perceives the world. For adult humans, fortunately, language allows psychologists to simply ask a person how they feel or what they know. But for subjects who can't speak articulately—or at all—researchers have to get more creative.

In the 1980s, psychologists devised a strategy to see whether young children were aware of the thoughts and perceptions of others, known as testing “false beliefs.” There are variations, but the test usually takes the form a simple scenario: The child is shown an object being put in a location by another person, who then leaves the room. While the first person is gone, a second person will move the object to different location. The child will then be asked to indicate where the first person will look for the object.

The child knows where the object really is now. But to answer the question correctly, he or she must assume that the first person still has a "false belief" about where the object is because they didn't see it getting moved. To psychologists, this proves that the child knows that other people can think differently than they do, and thus have a grasp of "theory of mind."

While the original studies involved children old enough to speak, more recent studies of "false beliefs" have looked at toddlers and even infants. In 2009, Buttelmann published research with a test showing that infants as young as 16 months old could recognize false beliefs in others. Testing this research in children too young to speak made Buttelmann wonder whether the same test could be used for other animals—namely, our close ape ancestors.

For the study, Buttelmann and his coauthors trained chimpanzees, bonobos and orangutans to help a person unlock two boxes, one of which had an object placed in them. (Initially Buttelmann worried that his subjects might tire of the task, but, he recalls, “they had fun—I've never experienced such motivated [subjects] before."

The researchers then introduced the actual test. First, a research assistant placed an object in one of the two boxes, with a second person then moving the object to the other box. In one experiment, the first person would remain in the room while this switch happened, and then go to open to the box they originally put the object in (the "true belief" experiment). In the second, the first person would be out of the room while the switch happened, and then go for the original box (the "false belief" experiment).

This illustration shows the experimenter trying to open a box, which may or may not have an object in it. The ape can choose to help the experimenter based on whether it thinks the person knows which box holds the object.
(Buttelmann et al / EurekAlert)

They found that the first person was more likely to receive help—in the form of the ape unlocking the correct box for them—when it appeared that the person had a "false belief" about which box their object was in.

By contrasting a "true belief" person with a "false belief" person, Buttelmann says his team was able to show that "it is their understanding of the experimenter" that leads the apes to choose which box they do. They're less likely to help a person who knows where the object is because they know that person isn't confused—or so the logic goes.

The thing is, these sorts of tests are always open to interpretation, says Robert Lurz, a philosopher at Brooklyn College who has done extensive research on false beliefs and animal cognition. Pointing a similar study last year on apes by some of Buttelmann's coauthors in this study, Lurz says that how to interpret the behavior of these apes is not a settled question yet.

"Even though these two studies converge, it is not clear that they converge on the hypothesis that great apes have an understanding of others’ false beliefs or on the hypothesis that great ape have an understanding of others’ perceptions and goals," says Lurz, who was not involved in the study.

In other words, the apes' actions don't necessarily prove that they're actually recognizing false beliefs in the experimenters. "They might just infer that the experimenter wants the object because she returns to the box where she last saw the object placed," he says. "That’s a pretty good reason to think that she wants the object."

At the same time, Lurz said he was impressed by how the researchers designed this kind of experiment. "It is very difficult to design valid theory-of-mind test for animals," he says. "And so I applaud [the study's] use of an innovative procedure for testing false-belief attribution in apes."

What would be the evolutionary purpose of recognizing false beliefs? Buttlemann has some ideas. One example, he says, is that a male could perceive that the group's dominant male doesn't know that his favorite female is not where he thinks she is. The first male could then take advantage of the dominant male's false belief to mate with the female—thus increasing the likelihood of passing on his genes.

But that’s just a hypothetical scenario. For future research, Buttelmann plans to redesign his test to look at other members of the animal kingdom and get a better sense of how and why theory of mind evolved. "I would love to find out what factor might be the factor that drove the evolution of theory of mind," he says.